1. Field
The present invention relates generally to the inspection of nuclear fuel rods and, more particularly, to the inspection of a nuclear fuel pellet stack within a hermetically sealed fuel rod cladding to detect missing pellet surfaces and pellet-to-pellet gaps.
2. Related Art
The large nuclear reactors utilized for power generation employ an array of a large number fuel rods containing nuclear fuel. Each rod comprises a metal tube or a sheath which may be from 8 to 15 feet (2.4-4.6 m) long and up to one-half inch (1.27 cm) in diameter, and which contains a stack of cylindrical fuel pellets of suitable fissionable material such as uranium oxide. The upper end of the tube is empty of fuel pellets and forms a plenum for a gas or other fluid under substantial pressure which fills the top of the rod and also a small clearance space around the fuel pellets. The fuel rods are supported in parallel groups in fuel assemblies which may typically contain upwards of 300 fuel rods, and the complete nuclear reactor is made up of a large number of these fuel assemblies arranged in a suitable configuration in an active core.
The metal tubes of the fuel rods, also known as cladding, constitute the primary containment boundary for the radioactive nuclear fuel, and inspection of the internal components of the rod that can affect the rod's integrity is of primary importance. In the manufacture of the fuel rods, the tubing itself and the end cap welds are carefully inspected and helium leak tested. Since a nuclear reactor may contain upwards of 40,000 fuel rods, a probability exists that some number of defective rods will be present even with a highly effective manufacturing quality control program. It is also desirable to inspect the fully loaded fuel pellet stack for defects such as missing pellet surfaces and pellet-to-pellet gaps which can ultimately compromise the cladding's integrity or affect core performance. The temperature differences on the outer cladding surface of an assembled fuel rod can result from differences in the radial thermal resistance between the cladding inside diameter and the fuel pellet outer surface due to a missing pellet surface or a pellet-to-pellet gap. It is important to detect conditions such as this that might ultimately result in breaches of the cladding which could lead to fission products leaking into the reactor coolant and can result in many conditions that increase operating costs. These conditions include: (1) high radiation readings in the primary cooling system; (2) increased volume of liquid radioactive waste; (3) increased volume of solid radioactive waste due to more frequent demineralizer bed replacement; (4) increased costs for disposal of spent fuel assemblies due to special handling and additional decontamination; and (5) increased exposure to personnel. These increased costs outweigh the costs incurred by testing assemblies. Once identified, a leaking fuel rod may be extracted from the fuel assembly and replaced with a dummy rod to allow the eventual reload of the assembly in the core. To the extent failure mechanisms can be located in advance of placing the fuel assemblies in the core, the costs of replacing defective rods can be minimized.
Accordingly, it is an object of this invention to provide a means of nondestructively inspecting a fuel pellet stack sealed within the cladding of a nuclear fuel rod.
Further, it is an object of this invention to provide such an inspection method that can be performed efficiently, with minimal effort and expense.